The present invention relates generally to output buffer design for integrated circuits, and more particularly an aspect of the invention relates to capacitive feedback circuits in output buffers for the control of transition rates.
Integrated circuits (ICs) are typically capable of operating internally at high speeds because the output loading of circuit stages, due to the parasitic resistances and capacitances inherent in the transistor and interconnect structures, is small. This tends to obviate the need for the use of complex internal buffer circuits. However, when signals must be communicated between the IC and external components, the magnitude of the output loading increases dramatically. Special buffer circuits are often used to communicate signals from inside to outside the IC in view of the heavy output loading. FIG. 1 shows a block level diagram of a prior art output buffer circuit configuration.
The data is sent to an output pad through a pre-driver circuit and either a pull up driver or pull down driver. In addition to being able to drive large loads, output buffers in electronic systems are also sometimes required to provide slew rate controlled signal transitions. Control over the slew rate or transition rate in Universal Serial Bus low speed output circuit applications is especially important because the wires connecting devices such as mouses, keyboards, etc. are typically unshielded. Transition rates with a high rate of change generate electromagnetic interference. The generated electromagnetic interference may distort, for example television reception, radio reception. Presently, output buffers designed to meet given slew rate specifications that minimize generate electromagnetic interference use a relatively large value feedback capacitor to implement slew rate control.
FIG. 2 shows a block diagram of an output buffer having an output stage, a pull-up pre-driver stage, a pull-down pre-driver stage and capacitively coupled feedback to both pull-up and pull-down pre-drivers. Unfortunately, large value capacitors consume correspondingly large amounts of chip area. Larger chip area leads to higher manufacturing costs because yield is closely related to chip area.
In a previously used technique, the output buffer for a USB output circuit requires a capacitor having a large capacitance value in order to pass enough current to achieve a desired edge rate. Transition rate is the charging potential or in other words rate of the change of voltage over time at a particular output. Transition rate is can be stated mathematically as dV/dT. The current through a capacitor can be calculated in accordance with the following equation, Ic=C*dV/dT. Capacitor current equals the capacitance value times the transition rate. Typically, transition rate may also be referred to as slew rate.